The answer to 'why is my furnace not working' is that there could be several common causes, such as a thermostat issue, a problem with the pilot light or ignition, or a mechanical failure. The symptoms you may notice include the furnace not turning on, poor airflow, strange noises, or the pilot light going out.
Some simple DIY troubleshooting steps you can try include checking the thermostat settings, replacing the air filter, and inspecting the pilot light. However, if the issue persists or you're not comfortable working on the furnace, it's best to call in a professional HVAC technician to diagnose and repair the problem.